WO1998006987A1

WO1998006987A1 - Improvements in refrigeration control

Info

Publication number: WO1998006987A1
Application number: PCT/AU1997/000522
Authority: WO
Inventors: Roderick Alan Hislop
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-08-15
Filing date: 1997-08-15
Publication date: 1998-02-19
Anticipated expiration: 1999-02-15
Also published as: AUPO169396A0

Abstract

A method for controlling the operation of a vapour-compression refrigeration system so that the total energy consumed by the system during operation is reduced, the method comprising the steps of: 1. measuring refrigeration load and at least one selected ambient condition; 2. calculating the total energy consumed by the compressor(s), evaporator(s), condenser(s) and fan(s) under the said refrigeration load at the said at least one selected ambient condition; and 3. adjusting the pressure or temperature set point for the condenser(s) and/or evaporator(s) whilst the system is operating so as to reduce the total energy consumed by this system having regard to the said refrigeration load and ambient condition.

Description

IMPROVEMENTS IN REFRIGERATION CONTROL

TECHNICAL FIELD -

THIS INVENTION relates to the control of refrigeration systems, and more particularly to a method for minimising the energy consumption of vapour- compression refrigeration systems during operation, especially under low load conditions

BACKGROUND ART -

In order to understand how to minimise the energy consumption of a vapour- compression refrigeration system, it is first necessary to understand how such refrigeration systems work

Vapour-compression refrigeration system generally consist of a compressor, two heat exchangers (a condenser and an evaporator), an expansion valve and a refrigerant The operation of a vapour-compression refrigeration system has a number of stages Firstly, the refrigerant enters the compressor as a low pressure gas at the evaporating (or suction) temperature wherein it is compressed into a high temperature, high pressure gas The hot refrigerant gas is then cooled to liquid form in the condenser before being pumped, or passing under its own pressure, across the expansion valve to the evaporator In passing through the expansion valve, the refrigerant changes into a low pressure, low temperature liquid which is then able to absorb heat in the evaporator Finally, the absorption of heat in the evaporator causes the liquid to boil into a gas before being compressed again

When the hot refrigerant gas is being cooled to liquid form in the condenser, heat is rejected into what is known as the "condensing medium" Whilst there are many different types of condensing media, the most commonly used is simply ambient air and/or ambient water. .Alternatively, the condensing medium may take the form of some other process liquid or gas transported by pumps or fans, respectively.

Most large refrigeration plants in Australia use either water cooled cooling towers or evaporative condensers for condensation.

In water cooled cooling towers, the cool water from the cooling tower is pumped around the condensing heat exchanger containing the refrigerant. As it does so, heat is rejected into the water. The amount of heat rejected is proportional to the water flow around the condenser. Increasing the water flow increases the rate of heat transfer from the refrigerant for a given ambient water temperature, and therefore reduces the condensing temperature.

Evaporative condensers, on the other hand, utilise the cooling powers of evaporating water in an air stream directly over the condensing heat exchanger containing the refrigerant as the method of heat rejection. The rate and amount of heat rejection is dependent on both the condensing temperature and the air flow rate about the condenser. Increasing the air flow increases the rate of heat transfer from the refrigerant, for a given ambient air temperature, and therefore reduces the condensing temperature.

The effect of reducing the condensing temperature can be explained theoretically by the following formula which gives the co-efficient of performance (COP) of an "ideal" refrigeration cycle (the Carnot cycle) wherein Te is the evaporating temperature in Kelvin and Tc is the condensing temperature in Kelvin.

Ma COP = Te

(Tc - Te) The "ideal" cycle does not exist in practice, with practical refrigeration cycles typically having a COP of between about 60% and 70% of that given by this formula. Similarly, real refrigeration systems are more efficient at heat rejection when operated at reduced condensing temperatures and also for raised evaporating temperatures.

Efforts to control the operation of vapour-compression refrigeration systems have up until now centered largely around controlling the operating parameters of the condenser with little or no regard to the energy consumed by the other components in the system. Typically, fixed temperatures are maintained for evaporating and condensing conditions regardless of the heat load or ambient conditions.

Whilst it holds true that reducing the condensing temperature (or pressure) does reduce the amount of compression required, the amount of work that needs to be done by the fans in order to achieving the reduced condensing temperature increases markedly.

Probably the most common method for controlling the operation of a vapour- compression refrigeration system, and more particularly the condenser, involves preselecting a discharge pressure or temperature for the refrigerant and maintaining that pressure or temperature at a constant level, known as a "set point", whenever the refrigeration system is operating. In practice, this involves measuring the pressure or temperature of the refrigerant and adjusting the working parameters of the condenser to suit. This method of control is known as the Constant Condenser Temperature (CCT) method. The CCT method is very cost effective for peak design conditions, typically peak summer conditions, but such conditions typically occur for only 1-2% of the time. At other times of the year or under lower load conditions, the condensing set point is too high and a great deal of energy is wasted. Even when the set point is reduced to a level below that used for peak summer design conditions, the system will only be operating at the optimum efficiency for the few specific ambient and refrigeration load conditions

Another commonly used method of controlling the operation of vapour- compression refrigeration systems is the Floating Condenser Temperature (FCT) method In this method, the condenser is operated continuously at full capacity, thereby minimising the amount of energy consumed by the compressor However, whilst this method can improve the operating efficiency of the refrigeration system under certain conditions, under low load conditions energy efficiency drops, largely because of the excessive energy expended by the fans Again, the system will only be operating at optimum energy efficiency for a few specific conditions (full load) For occasions with less than full load, the fans blowing the air about the condenser are likely to be operating excessively and thus wasting energy

Little or no effort has been expended in attempting to balance the competing energy requirements of the condensers, fans and/or pumps and compressor so as to optimise the energy efficiency of the overall system, during variable operating conditions For example, it is known that vapour-compression refrigeration systems are more efficient for lower condensing temperatures and for high evaporating temperatures (and pressures) because the pressure ratio of compression is reduced Typically the improvement in efficiency ranges from 1 5%/l°c to 3%/l°c for most refrigeration applications However, in order to reduce the pressure ratio, the evaporating or condensing equipment (or both) require increased energy input in order to reduce the condensing temperature For example in order to achieve a lower condensing temperature, the cooling tower fans need to operate at higher speeds, thus using more energy

It is therefore an object of the present invention to provide a method for controlling the operation of a vapour-compression refrigeration system so as to minimise the total energy consumed by the system under varying load conditions Whilst the current invention is described herein with particular reference to large refrigeration plants which use water cooled cooling towers or evaporative condensers for condensation, it will be readily understood by those skilled in the art that it has application to refrigeration systems which employ alternative condensing and evaporating means.

DISCLOSURE OF THE INVENTION:-

In a first aspect of the present invention, there is provided a method of controlling the operation of a vapour-compression refrigeration system so that total energy consumed by the system during operation is reduced, the method comprising the steps of-

1. measuring refrigeration load and at least one selected ambient condition;

2. calculating the total energy consumed by the compressors), evaporator(s), condenser(s) and fan(s) under the said refrigeration load at the said at least one selected ambient condition; and

3. adjusting the pressure or temperature set point for the condenser(s) and/or evaporator(s) whilst the system is operating so as to reduce the total energy consumed by this system having regard to the said refrigeration load and ambient condition.

The refrigeration load is determined by the required "heat rejection", that is, the amount of heat to be removed from the refrigerant, and also the ambient temperature. The ambient temperature is described in terms of the ambient dry bulb temperature when an air cooled condenser is used, and in terms of the ambient wet bulb temperature when a water cooled cooling tower or evaporative condenser is used. The Applicant has discovered that vapour-compression refrigeration systems work most efficiently when the temperature of the condenser cooling water is selected and maintained within a predetermined range relative to the ambient wet bulb temperature

Accordingly, in a second embodiment of the invention, there is provided a method for controlling the operation of a vapour-compression refrigeration system which includes a water cooled condenser (as hereinafter defined) so that the total energy consumed by the system during operation is reduced, the method comprising the steps of -

1 measuring (or calculating) the ambient wet bulb temperature,

2 measuring the temperature of the condenser water,

3 calculating the difference in temperature between the said ambient wet bulb temperature and the said condensing cooling water temperature ,

4 generating an output signal to drive a fan in order to adjust and/or maintain the said temperature of the condenser cooling water so that the resulting said difference in temperature between said wet bulb temperature and said condensing water temperature is maintained within a pre-determined range at which the total energy consumption by the said system is minimised

Similarly, the invention can be used to control the evaporating temperature and evaporator fan speed

The term "water cooled condenser" as used herein is intended to include any condenser which utilises the cooling properties of water, including direct water cooled condensers, condensers through which water from a cooling tower is pumped or evaporative condensers.

As used herein the term "approach temperature" is intended to mean the difference in temperature between the wet bulb temperature and the condensing water temperature for water cooled condensers and is intended to mean the difference in temperature between the dry bulb temperature and the condensing vapour temperature for air cooled condensers.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS -

The invention will now be described in more detail with reference to the following example and the accompanying drawings, wherein -

FIGURE 1 is a graph showing the operating efficiency for a Luke HRWC- 900C refrigeration system across a range of approach temperatures with consumption in kilowatts plotted on the Y axis and the approach temperature in degrees Celsius on the X axis and.

FIGURE 2 is a graph showing the operating efficiency of the same Luke HRWC-900C refrigeration system at an ambient wet bulb temperature of 16° Celsius under a range of refrigeration loads with water cooled cooling tower and condenser controlled by the Constant Condenser Temperature method, the Floating Condenser Temperature method, the Applicant's Adjustable Approach Temperature method with on/off speed drive fans (AAT on/off) and the Applicant's Adjustable Approach Temperature method with variable speed drive fans (AAT VSD) respectively, with total power consumption in kilowatts plotted on the Y axis and % refrigeration load on the X axis. FIGURE 3 is a graph showing the operating efficiency of the variable speed drive fans and on/off speed drive fans of a Luke HRWC-900C refrigeration system across a range of approach temperatures with power consumption in kilowatts plotted on the Y axis and the approach temperature in degrees Celsius on the X axis.

A series of trials were conducted to determine the optimum approach temperature for the system The energy consumed by the compressors and fans respectively was measured over a range of approach temperatures, the results of which appear in Figure 1. It was discovered that as the approach temperature decreased, the energy consumed by the fans increased, whereas the energy consumed by the compressors decreased.

When the total energy consumed by both the compressors and fans was calculated and plotted against the range of approach temperatures, it was discovered that the combined graph exhibited a marked dip for an approach temperature in the range of about 6° to 7° Celsius. In other words, the energy efficiency of the Luke chiller was greatest where the approach temperature was between 6° and 7° Celsius. It was also found that this optimum approach temperature was independent of refrigeration load and other ambient conditions for the Luke HRWC-900C chiller trialed. It is anticipated, however, that the optimum approach temperature or temperature range will vary between individual refrigeration systems.

Of course, the actual energy consumed by any system will vary depending on the refrigeration load, the characterises of the compressor, the amount of energy consumed by the condensing equipment and also the ambient conditions. However, the Applicant has made the discovery that the minimum energy consumed by a vapour-compression refrigeration system is related to the approach temperature with the minimum total energy being consumed for a similar approach temperature or range of temperatures regardless of other variables The Applicant then measured the energy consumed by the Luke HRWC-900C refrigeration system when controlled by the method of the current invention and compared it with the energy consumed by the same refrigeration system when controlled by existing methods. The results of these trials are illustrated in Figure 2 where total power consumption in kilowatts is plotted on the Y axis and the percentage refrigeration load is plotted on the X axis. The diamond-marked solid line represents the energy consumption of the Luke HRWC-900C refrigeration system when controlled by the Constant Condenser Temperature method. The square-marked dotted line represents the energy consumption of the Luke HRWC-900C refrigeration system when controlled by the Floating Condenser Temperature method. The triangle-marked dotted line represents the energy consumption of the Luke HRWC-900C refrigeration system when controlled by the Applicant's Adjustable Approach Temperature method with on/ofF speed drive fans. And the cross-marked dotted line represents the energy consumption of the Luke HRWC-900C refrigeration system when controlled by the Applicant's Adjustable Approach Temperature method with variable speed drive fans.

From Figure 2 it can be seen that at an ambient wet bulb temperature of 16° Celsius under 100% refrigeration load there is little or no difference in energy consumption between Floating Condenser Temperature method and the Applicant's method. However, as the refrigeration load is reduced, the improvements in energy efficiency conferred by the Applicant's method are more marked. Thus, under peak summer time conditions with refrigeration loads approaching 100%, there is little or no improvement in energy efficiency between the Floating Condenser Temperature method and the Applicant's method. However, with reduced refrigeration loads and/or reduced ambient temperatures, there is a marked improvement in the energy efficiency of this system when controlled by the Applicant's method. From Figure 2 it can also be seen that the Applicant's method, under the complete range of refrigeration loads, is more energy efficient than the Constant Condenser Temperature method .

When the energy consumed by the Luke HRWC-900C refrigeration system controlled by the Applicant's Adjustable Approach Temperature method with on/off speed drive fans, was compared with that consumed by the same refrigeration system controlled by the Applicant's Adjustable Approach Temperature method with variable speed drive fans it was discovered that energy efficiency improved further when variable speed drive motors were used to adjust the air flow. This conclusion may be drawn from Figure 2, but is more apparent in Figure 3 where the results of trials testing the operating efficiency of the evaporating fans of the refrigeration system across a range of approach temperatures have been plotted. In Figure 3, the square-marked solid line indicates the power consumption of evaporating fans with variable speed drive motors and the diamond-marked solid line indicates the power consumption of evaporating fans with simple on/off speed drive motors.

Whilst the method as described herein may be implement in a variety of ways, the Applicant has found that the best method of performing the invention is using a computer program designed to monitor and control the various components of a vapour-compression refrigeration system in accordance with the method described herein.

It will, of course, be realised that whilst the above has been given by way of illustrative example, modifications may be made to the above method by persons skilled in the art without departing from the broad scope and ambit of the invention described.

Claims

CLAIMS -

1. A method for controlling the operation of a vapour-compression refrigeration system so as to decrease energy consumption; said vapour-compression refrigeration system comprising at least one compressor, at least one evaporator, at least one condenser and at least one fan; said at least one condenser and evaporator each having pressure and temperature set points; said method comprising the steps of

(a) measuring or calculating the refrigeration load and at least one selected ambient condition;

(b) calculating the total energy consumed by each compressor, evaporator, condenser and fan operating under said refrigeration load at said at least one selected ambient condition; and

(c) adjusting the pressure or temperature set point of said at least one condenser and/or evaporator whilst the system is operating so as to reduce the total energy consumed by the vapour-compression refrigeration system having regard to the said refrigeration load and said selected ambient condition.

2. A method for controlling the operation of a vapour-compression refrigeration system according to Claim 1 in which said condenser is a water cooled cooling tower or evaporative condenser employing condenser cooling water.

A method for controlling the operation of a vapour-compression refrigeration system according to Claim 2 in which said selected ambient condition is the ambient wet bulb temperature

A method for controlling the operation of a vapour-compression refrigeration system according to Claim 1 in which said condenser is an air cooled condenser employing condensing vapour

A method for controlling the operation of a vapour-compression refrigeration system according to Claim 4 in which said selected ambient condition is the ambient dry bulb temperature

A method for controlling the operation of a vapour-compression refrigeration system according to Claim 3 in which the step of adjusting the pressure or temperature set point of said at least one condenser and/or evaporator whilst the system is operating so as to reduce the total energy consumed by the vapour-compression refrigeration system having regard to the said refrigeration load and said selected ambient condition is achieved by -

(a) measuring the temperature of the condenser cooling water,

(b) calculating the difference in temperature between the said ambient wet bulb temperature and the temperature of the condenser cooling water,

(c) generating an output signal to drive a fan in order to adjust and/or maintain the said temperature of the condenser cooling water so that the resulting said difference in temperature between said wet bulb temperature and said condensing cooling water temperature is maintained within a pre-determined range at which the total energy consumption by the said system is minimised.

7. A method for controlling the operation of a vapour-compression refrigeration system according to Claim 5 in which the step of adjusting the pressure or temperature set point of said at least one condenser and/or evaporator whilst the system is operating so as to reduce the total energy consumed by the vapour-compression refrigeration system having regard to the said refrigeration load and said selected ambient condition is achieved by

(a) measuring the temperature of the condensing vapour;

(b) calculating the difference in temperature between the said ambient dry bulb temperature and the temperature of the condensing vapour;

(c) generating an output signal to drive a fan in order to adjust and/or maintain the said temperature and or flow rate of the condensing vapour so that the resulting said difference in temperature between said dry bulb temperature and said condensing vapour temperature is maintained within a pre-determined range at which the total energy consumption by the said system is minimised.

8. A method for controlling the operation of a vapour-compression refrigeration system according to any one of the preceding claims as performed with the aid of a computer program.

9. A method for controlling the operation of a vapour-compression refringeration system accofrding to any one of Claims 1 to 7 as performed with the aid of an electrical or mechanical device.

10. A method for controlling the operation of a vapour-compression refrigeration system according to any one of the preceding claims in order to achieve improved energy efficiency, particularly under low load conditions